Revenue (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue from Contracts with Customer [Line Items]
Cumulative effect on retained earnings			$ 2,000
Accounts receivable, net	$ 324,911		324,911			$ 304,174
Contract assets	11,000		11,000		$ 7,800
Contract liabilities	$ 2,900		2,900			$ 2,100
Revenue recognized from contract liability during the period			2,100
Average transit time	3 days
Revenues	$ 629,735	$ 528,643	1,811,549	$ 1,549,372
Operating expenses	566,349	492,769	1,662,265	1,450,613
Difference between revenue guidance in effect before and after Topic 606
Revenue from Contracts with Customer [Line Items]
Revenues	2,200		3,200
Operating expenses	1,500		1,900
Other revenues
Revenue from Contracts with Customer [Line Items]
Revenues	15,739	$ 16,613	44,958	$ 48,796
Other revenues | Difference between revenue guidance in effect before and after Topic 606
Revenue from Contracts with Customer [Line Items]
Revenues	$ (3,600)		$ (10,700)